This invention relates to the debismuthizing of lead.
The applicant is aware of the Kroll process for removing bismuth from lead. This process involves the addition of approximately equal quantities of magnesium and calcium to the molten lead at 420.degree. C., the slow cooling of the lead and removal of the dross formed; several drossings being carried out; and finally, the cooling of the lead almost to freezing point and the removal of the bismuth-rich crust from the surface.
Lead recovered from the crust is returned to the circuit and the bismuth recovered is purified.
The success of Kroll's process resulted from the fact that calcium and bimuth react to form calcium bismuthide Ca.sub.3 Bi.sub.2 which has a high melting point and is fairly insoluble in lead.
The presence of magnesium further decreases the solubility of Ca.sub.3 Bi.sub.2. Magnesium itself forms a high melting point bismuthide Mg.sub.3 Bi.sub.2.
J. O. Betterton also did work on the debismuthizing of lead and a process very similar to Kroll's using calcium and magnesium is still in use in many parts of the world to this day. This process is known as the Betterton-Kroll and is probably the most favoured method in use at present. This process takes the bismuth content down to approximately 0.01% or 0.005% at best.
The Jolivet Process was developed to refine lead to bismuth levels as low as 10 parts per million or 0.001%. This was achieved by using potassium, usually after most of the bismuth had been removed by the use of calcium and magnesium.
Some refineries have considered potassium too dangerous a metal to store or work with and have abandoned the Jolivet process in favour of the Betterton-Kroll process.
Experiments have also been carried out with sodium and antimony, but only the use of antimony seems to have found commercial use. At some refineries antimony is used in the final stages to take bismuth down to low levels.
An electrolytic process completely different in character from any of the preceeding was developed by Betts. The Betts process involves the electrolytic refining of lead containing bismuth and other impurities using as an electrolyte a solution of lead fluorosilicate and free fluorosilicic acid, the bismuth and other impurities remaining at the anode as a sludge. In reality it is a method of removing lead from its impurities in that lead is removed from an anode and deposited as pure lead on a cathode, as opposed to removing impurities from lead.
Finally, it should be noted that many variations of the Betterton-Kroll process are in use today but, in general, the original technology is still being used, and the debismuthizing of lead remains a relatively expensive operation.